Because of an antenna unfriendly neighborhood and limited space I am
considering using a Diamond antenna either with or without radials,
specifically X-50 vs SE-100.
They are the same length, but the SE-100 is a "marine" antenna without
radials. Nominally they both sport the same gain etc.

What are the tradeoffs?
Without radials it would certainly look less conspicious.

TIA

Marc

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In article ,
Marc Heusser d wrote:

Because of an antenna unfriendly neighborhood and limited space I am
considering using a Diamond antenna either with or without radials,
specifically X-50 vs SE-100.
They are the same length, but the SE-100 is a "marine" antenna without
radials. Nominally they both sport the same gain etc.

What are the tradeoffs?
Without radials it would certainly look less conspicious.

In an antenna of this sort, I believe that the primary function
of the radials is to act as a sort of feedline choke... they minimize
any tendency of RF coming up the inside of the feedline shield to flow
back down along the outside of the feedline. Such RF on the feedline
can cause radiation which will disturb the pattern of the antenna to
some extent.

A radial-less version of the antenna has no such choking, and there
might be some RF flow on the feedline as a result. This might cause a
slight reduction in gain in some directions, due to changes in the
antenna pattern.

If there's a *lot* of RF current flow on the outside of the feedline,
it might tend to couple into nearby conductors, and might (in some
installations) be the cause of QRM (e.g. TVI).

Is this likely to present a problem? In most installations I'd tend
to doubt it. If it is, you might be able to correct it by (1)
insulating the antenna from its mast, and (2) adding some sort of
choke to the feedline, just below the antenna feedpoint... either wind
a foot or two of the coax into a coil, or clamp a ferrite or two
around the coax.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Do what you wish but the ground radials on the diamond A50 are there for a reason. It would be in your best interest to leave them on. It's small enough to begin with that you should not have any problem hiding it. I have one up and it's well hid at 30'.
N9ZAS

On Tue, 20 Oct 2009 16:31:28 -0700, (Dave Platt)
wrote:

In an antenna of this sort, I believe that the primary function
of the radials is to act as a sort of feedline choke... they minimize
any tendency of RF coming up the inside of the feedline shield to flow
back down along the outside of the feedline.

Hi Dave,

I've modeled the conventional 4 radial GP, in free space; and, if
anything, the down lead (added as a 1/4 wave drop wire) supports more
current than the "radiator."

When I did the same thing with a ground plane of 120 radials (and the
added drop wire), the down lead supported nearly as much current as
the "radiator."

Neither radial configuration exhibits any/much choking.

In essence, these models were simply vertical dipoles with a star of
neutral elements. Of course, changing the length of the drop wire
complexifies the outcome; and would, I suspect, merely mimic nature.

Going further by following up on that last thought:

When for both radial configurations I vary the length of the down lead
(starting at 0.1 then adding 0.05 wl increments), the down lead shows
the current distribution of any inverted radiator that correlates to
that length (up to and including 3/4ths). However, the drive point
Z's R varies considerably ONLY around resonances (quarter and three
quarter wl) while the Z's X barely shifts value except (and again) at
those resonances.

What the elevated radials appear to do is dominate (buffer) the feed
point Z against variations of a feedline's length. Without these
radials, then this would be nothing other than a vertical Off Center
Dipole - if the feedline doesn't find ground, that is. The presence
of ground takes all bets off the table, but I don't think ground will
introduce choking action to the list of qualities for elevated
radials.

73's
Richard Clark, KB7QHC

On Oct 20, 6:31*pm, (Dave Platt) wrote:
In article ,
Marc Heusser id wrote:

Because of an antenna unfriendly neighborhood and limited space I am
considering using a Diamond antenna either with or without radials,
specifically X-50 vs SE-100.
They are the same length, but the SE-100 is a "marine" antenna without
radials. Nominally they both sport the same gain etc.

What are the tradeoffs?
Without radials it would certainly look less conspicious.

In an antenna of this sort, I believe that the primary function
of the radials is to act as a sort of feedline choke... they minimize
any tendency of RF coming up the inside of the feedline shield to flow
back down along the outside of the feedline. *Such RF on the feedline
can cause radiation which will disturb the pattern of the antenna to
some extent. *

A radial-less version of the antenna has no such choking, and there
might be some RF flow on the feedline as a result. *This might cause a
slight reduction in gain in some directions, due to changes in the
antenna pattern.

If there's a *lot* of RF current flow on the outside of the feedline,
it might tend to couple into nearby conductors, and might (in some
installations) be the cause of QRM (e.g. TVI).

Is this likely to present a problem? *In most installations I'd tend
to doubt it. *If it is, you might be able to correct it by (1)
insulating the antenna from its mast, and (2) adding some sort of
choke to the feedline, just below the antenna feedpoint... either wind
a foot or two of the coax into a coil, or clamp a ferrite or two
around the coax.

--
Dave Platt * * * * * * * * * * * * * * * * * AE6EO
Friends of Jade Warrior home page: *http://www.radagast.org/jade-warrior
* I do _not_ wish to receive unsolicited commercial email, and I will
* * *boycott any company which has the gall to send me such ads!

I suppose this guy is trying to be funny when he talks about radials
choking the current flowing on the outside of the coax. that's the
job for Baluns.

Radials are installed on vertical antennas to supply return RF current
flow back up to the other half (the vertical) of the antenna. Without
radials, you only have half an antenna. The placement, length, and
number of radials have a very definite effect not only on the resonant
frequency, but also on the SWR attainable.